This patent relates to a pressure relief valve for a packaging container. More particularly, this patent relates a preformed membrane type pressure relief valve for use with roasted coffee packages and the like.
The quality of roasted coffee beans or ground roast coffee will degenerate when exposed to oxygen. Commercial packagers have attempted to minimize the amount of oxygen in the package by using hermetically sealed soft packages or vacuum packed rigid packaging. However, soft packages can become bloated due to the carbon dioxide given off by the roasted beans or ground roast coffee after the roasting process. Rigid packaging will develop an internal pressure that will cause ground coffee to spray or aerosol when the package is opened. This creates a condition that can injure the person opening the package.
Overpressure relief valves may be used to eliminate the internal pressure of the carbon dioxide in all types of coffee packaging. The two most common types of overpressure relief valves used on roast coffee packaging are flexible membrane valves and rigid injection molded plastic type valves.
Membrane type overpressure relief valves generally comprise a flexible membrane or valve element adhered to a substrate by a thin film of pressure sensitive adhesive. In U.S. Pat. No. 4,134,535, the valve element is an oil-impregnated porous material disposed between a container wall and a foil covering. Both the container wall and the covering having openings therein, and pressure is relieved when gas in the container ruptures the oil in the valve element pores.
Buchner et al. U.S. Pat. No. 4,653,661 discloses a membrane valve comprising a plastic foil membrane having parallel adhesive strips. When the internal package pressure exceeds a certain level, the membrane bulges out so gas can escape through a middle path between the strips. A film of oil may be added to help keep the membrane in sealing contact with the package wall.
Domke U.S. Pat. No. 5,236,777 discloses a membrane type valve similar to that in Buchner ""661, but with wedge-shaped adhesive strips to prevent ambient air from entering the package through channels formed in the regions of transition between the adhesive-free zone and the peripheral adhesive areas.
Like membrane type valves, rigid injection molded plastic type valves (a.k.a. button valves) generally have a moveable valve element and a base, but the two usually are housed inside an injection molded plastic housing. In one button type pressure relief valve disclosed in U.S. Pat. No. 3,799,427, a rubber disk serving as the valve element rests on a valve seat within a plastic housing. A thin film of silicon oil coats the adjoining surfaces of the valve element and valve seat. The valve element lifts off the valve seat when the force caused by the internal package pressure exceeds the adhesion force of the viscous oil layer between the valve element and the valve seat.
Egli U.S. Pat. No. 4,365,715 discloses a plastic button type relief valve comprising a flexible plastic valve element mounted inside a housing. The valve is biased by mechanical means against the base of the housing.
Hollenstein U.S. Pat. No. 4,444,219 discloses a button type relief valve comprising a housing, a valve member and a xe2x80x9ckeeperxe2x80x9d designed to hold the valve member in place against the bottom of the housing. Oil held in grooves assists in keeping the valve element seated.
Tani U.S. Pat. No. 6,089,271 discloses a button type valve comprising a flexible, concave valve member and a xe2x80x9cpressing memberxe2x80x9d within a housing. The valve member forms a suction fit over an opening in the housing. Pressing member provides a mechanical assist to help the valve member remain sealed.
Membrane type valves tend to be less expensive and less bulky than button type valves. Yet a disadvantage of membrane type valves is that the valve element does not always close after the pressure inside and outside the package nears equilibrium. This is because most membrane type valves rely entirely on oil attraction forces to reseat the valve element onto the valve seat.
Thus it is an object of the present invention to provide a formed plastic film membrane type valve that reliably closes when pressure inside and outside the package nears equilibrium. As described below, this is accomplished by providing a membrane type pressure relief valve having a pre-stressed overlay that acts as a mechanical assist means to close the valve.
Further and additional objects will appear from the description, accompanying drawings, and appended claims.
The invention is a formed plastic film membrane type overpressure relief valve for packaging containers, especially those susceptible of internal gas build up such as roasted coffee packages. In one embodiment, the valve has a base sheet that serves as the valve seat, a movable flat valve element, and a plastic formed film overlay. The base sheet has an opening and can be affixed to a package wall by adhesive or other means. The opening in the base sheet is aligned with an opening in the package wall. The overlay has formed therein a raised annular portion that has a plurality of openings arranged around its apex. The valve element raises and lowers in response to the pressure differential between the inside and outside of the package, thereby opening and closing the valve. Oil may be used to increase the adhesion between the valve element and the base sheet of the valve.
The plastic film overlay is manufactured in an unstressed condition. In this unstressed condition, a central portion of the overlay extends below the plane defined by the perimeter portion of the overlay. When the valve is affixed to the base sheet, the center portion is raised into alignment with the plane of the perimeter portion and the raised annular portion deforms slightly. In this deformed, pre-stressed condition, the raised annular portion applies a downward force on the moveable membrane, biasing the valve element in the closed position.
The valve is affixed to the package such that the membrane seals off the opening in the package. The moveable valve element is held in place by the pre-stressed overlay and by the optional oil layer between the valve element and the base sheet. When the pressure inside of the container increases to a predetermined level, the valve element lifts off the base sheet, allowing gas inside the container to escape through the opening in the base sheet and out through the openings in the raised annular portion. When the inside and outside pressures near equilibrium, the valve element is reseated on the base by the oil attraction force with a mechanical assist from the annular portion.
In a second embodiment, the plastic formed film overlay has a raised center portion inside the raised annular portion. The raised center portion increases the mechanical assist properties of the film overlay, thereby providing a further mechanical assist in biasing the valve element in the closed position. In a third embodiment, the film overlay has an asymmetrical profile that prevents inadvertent valve closure when the valve contacts another package or object.
The valve may be used with or without a base sheet. If no base sheet is used, the valve is affixed directly to the package wall.